Polymers are long chains of organic molecules, which are assembled from many small repeating units, the monomers. The properties of polymers are very varied and can be tuned for specific applications by properly selecting the monomer identities, as well as their configuration and microstructure.
It is known to use chemical vapor deposition, CVD, methods to form an organic coating on a substrate. CVD techniques allow for forming conformal thin films on a variety of substrate materials having a variety of geometries. The deposited films are substantially free of impurities such as solvents, initiators or plasticizers used in wet chemistry. CVD of organic coatings is usually accomplished by thermal or plasma activated processes. It is further known that variants of CVD, such as initiated chemical vapor deposition, iCVD, or oxidative CVD, oCVD, may under certain conditions be used to deposit polymer layers on a substrate. While such known processes generally generate good results, the conditions under which such processes are operated typically require a sealed and pressurized enclosure in the coating device. This is a drawback as it requires expensive and technically involved coating devices, which impede the practical applications of CVD processes in many cases.
Known plasma-enhanced chemical vapor deposition, PE-CVD, processes can be operated either at low or atmospheric pressure. They have been investigated for the deposition of organic coatings. High functional group retentions have been achieved from soft plasma discharges, including pulsed plasma discharges, which make the obtained organic coatings suitable for adhesion applications. However, the so-called plasma-polymers created during PE-CVD typically exhibit a highly branched and cross-linked three-dimensional structure composed of randomly recombined fragments, which distinguishes them from polymers synthesized by conventional methods. The complex structured plasma-polymers obtained by PE-CVD are ill suited for applications involving so-called smart functional layers, which allow for example to sense gas, which may be optically active or temperature sensitive, or which may be electrically conductive. Such functional layers require linear and regular polymer structures.
Atmospheric pressure plasma enhanced chemical vapor deposition techniques imply the use of high voltage alternating current, AC, such as for example in atmospheric pressure dielectric barrier discharge, AP-DBD. An AP-DBD process can operate at room temperature, and has been used for industrial applications such as thin film deposition, surface cleaning, sterilizing or decontaminating, wettability or adhesion enhancement. According to the prior art, regular polymer layer deposition has not been achieved using AP-DBD processes. AC AP-DBD processes rely on current discharges of several tens of microseconds, which induce a large number of random side cross-linking reactions and lead to the formation of numerous new chemical groups. Under the electron bombardment and the current channels known as filaments, pinholes and heterogeneities are formed in the deposited thin films. Such known processes are therefore hard to control, especially for obtaining the regular structures required in functional polymer thins films.
It is an objective of the present invention to propose a method for forming polymer thin films which alleviates at least some of the drawbacks that are present in the prior art.